Multi-Cell Power System Controller

ABSTRACT

Various apparatuses and methods for detecting cell connection status of a multi-cell battery are disclosed herein. For example, some embodiments provide an apparatus for detecting cell connection status of a multi-cell battery. The apparatus includes a battery cell input for each cell, a cell connection status detector for each cell, and at least one comparator. Each of the cell connection status detectors is connected to a battery cell input and has a current-based status indicator output. The at least one comparator is connected to the current-based status indicator outputs. Each of the plurality of cell connection status detectors floats in a different supply voltage range. The at least one comparator is referenced to a lower voltage potential than at least one of the plurality of cell connection status detectors.

BACKGROUND

Batteries used in electronic devices are often made with multiple cells,each having a lower voltage than the entire battery, which are connectedin series to provide the desired voltage for the battery. For example, a12V battery may include ten 1.2V cells stacked in series to add up tothe desired output voltage. Because modern batteries have a very highenergy density, they present a danger of overheating and explosion ifdefective or if not used correctly. Devices powered by batteries ofteninclude monitoring circuits to protect the device and/or the battery.Various monitoring circuits may detect conditions such as defectivebatteries generating too much or too little current or voltage,defective electrical connections between a battery and a device, the useof improper battery chargers supplying too much current, etc. Monitoringcircuits may be used to warn a user or to turn off the device.

Power supply monitoring circuits designed for a single voltage input orsingle cell battery are not as effective at protecting devices andbatteries when the battery includes multiple cells. For example, twoadjacent cells in the battery may short together, creating a potentiallyhazardous condition, while the overall battery voltage remainssubstantially unchanged. In this case, a monitoring circuit designed fora single cell battery would monitor only the overall battery voltage andwould not immediately detect the hazardous internal short circuit.

The design of monitoring circuits for multi-cell batteries iscomplicated by the fact that each cell in a stack operates at adifferent voltage level. For example, in the ten cell 12V battery above,the output of the bottom cell when fully charged is 1.2V, and the outputof the ninth cell up the stack is 10.8V. Thus, monitoring circuits musttypically be customized for the various voltage levels in a stack ofcells in a multi-cell battery.

SUMMARY

Various apparatuses and methods for detecting cell connection status ofa multi-cell battery are disclosed herein. For example, some embodimentsprovide an apparatus for detecting cell connection status of amulti-cell battery. The apparatus includes a battery cell input for eachcell, a cell connection status detector for each cell, and at least onecomparator. Each of the cell connection status detectors is connected toa battery cell input and has a current-based status indicator output.The at least one comparator is connected to the current-based statusindicator outputs. Each of the plurality of cell connection statusdetectors floats in a different supply voltage range. The at least onecomparator is referenced to a lower voltage potential than at least oneof the plurality of cell connection status detectors.

In an embodiment of the apparatus, each of the cell connection statusdetectors comprises a bandgap referenced voltage to current converterconnected between a positive terminal and a negative terminal. Thepositive terminal and negative terminal comprise adjacent battery cellinputs. The at least one comparator is adapted to determine whether acurrent is on or off in the plurality of current-based status indicatoroutputs.

In an embodiment of the apparatus, each of the bandgap referencedvoltage to current converters includes a voltage divider that isconnected between the positive terminal and the negative terminal, abandgap referenced comparator that is connected to the voltage divider,and a cascoded current source connected to the output of the bandgapreferenced comparator. The bandgap referenced comparator is adapted toturn on an output when the voltage divider reaches a bandgap voltage.

In an embodiment of the apparatus, each of the bandgap referencedcomparators comprise includes a first diode-connected P-channeltransistor connected to the positive terminal, and a first NPN BJTtransistor connected to the diode connected P-channel transistor andhaving a base connected to the voltage divider. The bandgap referencedcomparators also include a second P-channel transistor connected to thepositive terminal. The gate of the second P-channel transistor isconnected to the gate of the first diode-connected P-channel transistor.The bandgap reference comparators also include a second NPN BJTtransistor connected to the second P-channel transistor. The base of thesecond NPN BJT transistor is connected to the base of the first NPN BJTtransistor. The emitter of the second NPN BJT transistor is connected tothe emitter of the first NPN BJT transistor through a second voltagedivider.

In an embodiment of the apparatus, each of the cascoded current sourcesincludes a pair of P-channel transistors, the first having a sourceconnected to the positive terminal and a gate connected to the output ofthe bandgap referenced comparator, the second with the source connectedto the drain of the first P-channel transistor and a gate connected tothe base of the first NPN BJT transistor. The second of the pair ofP-channel transistors in the cascoded current sources has a highervoltage rating than the first.

In an embodiment of the apparatus, each of the voltage dividers includesthree resistors connected in series between the positive terminal andthe negative terminal. The first resistor is connected to the positiveterminal. The bandgap referenced comparator is connected to a nodebetween the first resistor and the second resistor. Each of the cellconnection status detectors includes a resistor switchably connectedbetween the positive terminal and a node between the second and thirdresistors to change the voltage divider ratio.

In an embodiment of the apparatus, each of the cell connection statusdetectors is adapted to detect an overvoltage condition at theassociated one of the battery cell inputs when the resistors aredisconnected from the voltage dividers and to detect an undervoltagecondition when the resistors are connected to the voltage dividers.

In an embodiment of the apparatus, each of the cell connection statusdetectors includes a voltage to current converter connected between apositive terminal and a negative terminal, wherein the positive terminaland negative terminal comprise adjacent battery cell inputs. The atleast one comparator is adapted to determine a current magnitude in eachof the current-based status indicator outputs.

An embodiment of the apparatus is adapted in a first mode of operationto detect that a battery cell is connected to each of the plurality ofbattery cell inputs when an associated current-based status indicatoroutput is greater than a first current level. The apparatus is alsoadapted in a second mode of operation to detect an open condition ineach of the battery cell inputs when an associated current-based statusindicator output is less than a second current level.

In an embodiment of the apparatus, the at least one comparator includesan adjustable reference current source, and the apparatus is adapted togenerate the first current level in the adjustable reference currentsource in the first mode of operation and to generate the second currentlevel in the adjustable reference current source in the second mode ofoperation.

In an embodiment of the apparatus, the at least one comparator includesa single bandgap device and a plurality of current mirrors, with acurrent mirror for each of the current-based status indicator outputs.

In an embodiment of the apparatus, each of the voltage to currentconverters also includes a first diode connected P-channel transistorconnected between the positive terminal and the negative terminal, and asecond P-channel transistor connected between the positive terminal andthe current-based status indicator output. The gate of the first diodeconnected P-channel transistor is connected to the gate of the secondP-channel transistor.

In an embodiment of the apparatus, each of the voltage to currentconverters also includes a switch connected in series with the firstdiode connected P-channel transistor between the positive terminal andthe negative terminal.

In an embodiment of the apparatus, each of the voltage to currentconverters also includes a resistor connected in series with the switchand the first diode connected P-channel transistor between the positiveterminal and the negative terminal.

Other embodiments provide a method for detecting cell connection statusof a multi-cell battery. The method includes converting a voltage acrosseach cell of the multi-cell battery to a current to generate acurrent-based connection status indicating signal for each of the cells.The method also includes evaluating a current level of each of thecurrent-based connection status indicating signals. The method alsoincludes generating a common reference status indicating signal for eachof the current-based connection status indicating signals based on thecurrent levels. The common reference status indicating signals arereferenced to the same voltage level, while each cell of the multi-cellbattery is referenced to a different voltage level.

In an embodiment of the method, converting the voltage across each cellto a current includes applying a fraction of the voltage across eachcell to a bandgap device, and generating the common reference statusindicating signal includes asserting overvoltage indicator signals whencurrent is on in the current-based connection status indicating signals.

In an embodiment of the method, converting the voltage across each cellto a current also includes applying a different fraction of the voltageacross each cell to the bandgap device, and generating the commonreference status indicating signal also includes asserting undervoltageindicator signals when current is off in the current-based connectionstatus indicating signals.

In an embodiment of the method, generating the common reference statusindicating signal includes asserting a cell connected indicator signalwhen a current level in each of the current-based connection statusindicating signals is greater than a reference current level.

In an embodiment of the method, generating the common reference statusindicating signal also includes asserting an open circuit indicatorsignal when a current level in each of the current-based connectionstatus indicating signals is less than a second reference current level.

This summary provides only a general outline of some particularembodiments. Many other objects, features, advantages and otherembodiments will become more fully apparent from the following detaileddescription, the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the various embodiments may be realized byreference to the figures which are described in remaining portions ofthe specification. In the figures, like reference numerals may be usedthroughout several drawings to refer to similar components.

FIG. 1 depicts a multi-cell battery and multi-cell power systemcontroller in accordance with some embodiments.

FIG. 2 depicts a multi-cell battery and multi-cell power systemcontroller with bandgap referenced overvoltage and undervoltagedetectors in accordance with some embodiments.

FIG. 3 depicts a bandgap referenced overvoltage and undervoltagedetector in accordance with some embodiments.

FIG. 4 depicts a three cell battery connected to a three cell powersystem controller in accordance with some embodiments.

FIG. 5 depicts a two cell battery connected to a three cell power systemcontroller in accordance with some embodiments.

FIG. 6 depicts a multi-cell battery and multi-cell power systemcontroller with automatic cell configuration and open circuit detectorsin accordance with some embodiments.

FIG. 7 depicts voltage waveforms including cell voltages in a three cellpower system controller with automatic cell configuration when connectedto a two cell battery in accordance with some embodiments.

FIG. 8 depicts voltage waveforms including cell configuration outputsignals in a three cell power system controller with automatic cellconfiguration when connected to a two cell battery in accordance withsome embodiments.

FIG. 9 depicts a flow chart of a method for detecting cell connectionstatus of a multi-cell battery in accordance with some embodiments.

DESCRIPTION

The drawings and description, in general, disclose various embodimentsof a multi-cell power system controller for detecting cell connectionstatus of a multi-cell battery. For example, the power system controllermay be implemented in an integrated circuit that is connected to eachcell of a multi-cell battery, monitoring the connections to the cellsand performing any other additional functions as desired such as batterycharging and voltage or current regulation. The multi-cell power systemcontroller may be used with multi-cell batteries in devices such aspower tools, hybrid/plug-in electric vehicles, uninterruptible powersupplies (UPS), and other electronic devices, irrespective of the numberof cells in the battery.

Embodiments are disclosed that use a bandgap referenced device to detectovervoltage and undervoltage conditions at the cell connections that maybe caused by internal shorts in the cells or broken connections to thecells. Embodiments are also disclosed for automatic configuration of amulti-cell power system controller, detecting how many cells areconnected to the controller, and for configurable open circuit detectionat the cell connections. The various embodiments use current-basedstatus indicator signals, with connection status detectors for each cellfloating at the different voltage levels of the stacked cells in amulti-cell battery. The current-based status indicator signals from eachcell connection status detector are level shifted and interpreted in aground-referenced comparator circuit to generate status indicators thatmay be latched for use by other electronic circuits.

Turning now to FIG. 1, a multi-cell power system controller 10 isconnected to a battery 12 having multiple cells 14, 16 and 20. Each cell14, 16 and 20 is generally connected to the controller 10 through aresistor 22, 24, and 26, respectively, with a capacitor 30, 32 and 34connected across each cell to minimize noise. A cell connection statusdetector 40, 42 and 44 is connected across each cell 14, 16 and 20 inthe controller 10. Each cell connection status detector 40, 42 and 44has a current-based status indicator signal 50, 52 and 54, and acomparator 56 or group of comparators interprets the current-basedstatus indicator signals 50, 52 and 54 to generate common referencestatus indicator signals 60. Each of the cell connection statusdetectors 40, 42 and 44 operates across the voltage of its associatedcell. Because the cells 14, 16 and 20 are generally connected in series,the voltage level of each cell (e.g., 16) builds on the voltage level ofthe lower cell (e.g., 20). For example, in a three-cell 3.6V battery,three 1.2V cells 14, 16 and 20 are stacked. At full charge, the bottominput 62 is grounded at 0V relative to other inputs, the input 64 fromthe bottom cell 20 is at 1.2V, the input 66 from the middle cell 16 isat 2.4V and the input 70 from the top cell 14 is at 3.6V. Because eachconnection status detector 40, 42 and 44 is connected across one cell14, 16 and 18, respectively, the total voltage across each connectionstatus detector 40, 42 and 44 is the same. However, each connectionstatus detector 40, 42 and 44 is referenced to a different input 66, 64and 62, each with a different voltage level. The status of eachconnection is therefore signaled using current in the current-basedstatus indicator signals 50, 52 and 54, whether the presence or absenceof current or the current level. The comparator 56 interprets thecurrents in the current-based status indicator signals 50, 52 and 54 andgenerates the common reference status indicator signals 60, which arereferenced to the ground at the bottom input 62. The common referencestatus indicator signals 60 may be latched in the controller 10 for useby other circuits in the controller 10 or elsewhere, such as batterycharger circuits, etc.

Turning now to FIGS. 2 and 3, an embodiment of a controller 10 will bedescribed that uses a bandgap referenced device in each cell connectionstatus detector 40 to detect overvoltage and undervoltage conditions atthe cell connections. Overvoltage conditions may arise, for example, dueto short circuits between cells in a multi-cell battery 12, andundervoltage conditions may arise due to broken connections between thebattery terminals and pins of the integrated circuit housing thecontroller 10. The use of current-based status indicator signals 50, 52and 54 enables identical cell connection status detectors 40, 42 and 44to be used, irrespective of the number of cells 14, 16 and 20 in thebattery 12 or of the voltage between the inputs (e.g., 70 and 66) to acell (e.g., 40) and the bottom input 62. In other words, the cellconnection status detectors 40, 42 and 44 are fully differential withcurrent outputs enabling them to be operated between cell voltagesrather than being ground referenced. The number of cell connectionstatus detectors 40, 42 and 44 in the controller 10 is scalable to anynumber of cells 14, 16 and 20 because signal processing is done in thecurrent domain. The cell connection status detectors 40, 42 and 44 arebandgap referenced and are stable across temperature, an importantfeature of a battery monitoring application when battery temperaturesmay increase. The cell connection status detectors 40, 42 and 44 userelatively little area in an integrated circuit, having only one bandgapdevice per cell which may be multiplexed for both overvoltage andundervoltage detection. The cell connection status detectors 40, 42 and44 may be configured to have an extremely low average currentconsumption, for example less than 1 μA, substantially irrespective ofthe number of cells in the battery 12.

Because the cell connection status detectors 40, 42 and 44 may beidentical, the detailed description will be focused on one of the cellconnection status detectors 40 illustrated in FIG. 3. The input 70 fromthe top cell 14 acts as a positive terminal to the cell connectionstatus detector 40 and the input 66 acts as a negative terminal. Thecell connection status detector 40 includes a voltage divider 80 that isconnected between the positive terminal 70 and the negative terminal 66.A bandgap referenced comparator 82 is connected to the voltage divider80, and a cascoded current source 84 is connected to the bandgapreferenced comparator. The voltage divider 80 provides a fraction of thevoltage between the positive terminal 70 and negative terminal 66 to theinput 86 of the bandgap referenced comparator 82. When the voltage atthe input 86 reaches the bandgap voltage of the bandgap referencedcomparator 82, the output 90 of the bandgap referenced comparator 82 isturned on. Essentially, the bandgap referenced comparator 82 comparesthe voltage at the input 86 to the bandgap voltage of the bandgapreferenced comparator 82. The bandgap referenced comparator 82 turns onthe cascoded current source 84 to pull up the current-based statusindicator signal 50 and allow current to flow.

The voltage divider 80 may use any suitable devices and configuration toprovide a fraction of the voltage between the positive terminal 70 andnegative terminal 66 to the input 86 of the bandgap referencedcomparator 82. In one embodiment, the voltage divider 80 includes a topresistor 100, a middle resistor 102 and a bottom resistor 104 connectedin series between the positive terminal 70 and the negative terminal 66,with a node between the top resistor 100 and the middle resistor 102connected to the input 86 of the bandgap referenced comparator 82. Thisdivides the voltage across the cell, providing a fraction of the voltageacross the cell to the input 86 of the bandgap referenced comparator 82.

The cell connection status detector 40 may be adapted to operate in twomodes, one for detecting overvoltage conditions and the other fordetecting undervoltage conditions, by varying the ratio of the inputvoltage applied to the input 86 of the bandgap referenced comparator 82.To accomplish this, the voltage divider 80 may include another resistor106 connected by a switch 110 to the other resistors 100, 102 and 104,either at the node between the middle resistor 102 and the bottomresistor 104 or at the node between the top resistor 100 and the middleresistor 102, as desired, to switchably change the ratio of the voltagedivider 80. The fraction of the voltage between the positive terminal 70and negative terminal 66 provided to the input 86 of the bandgapreferenced comparator 82 may be varied by closing and opening the switch110.

The specific values of the resistors 100, 102, 104 and 106 in thevoltage divider 80 may be selected as desired based on the cell voltage,the voltage trip points selected as an overvoltage or undervoltagecondition, the voltage at the input 86 of the bandgap referencedcomparator 82 which turns on the output 90, etc.

The bandgap referenced comparator 82 includes a pair of NPN type bipolarjunction transistors (BJTS) 112 and 114, a pair of P-channel metal oxidesemiconductor (PMOS) transistors 116 and 120 and resistors 92 and 94.The bases of the BJTS 112 and 114 are connected together and form theinput 86 of the bandgap referenced comparator 82. The emitter of the BJT112 is connected to the negative terminal 66 through the seriesconnected resistors 92 and 94. The emitter of the BJT 114 is connectedto the emitter of the BJT 112 through the voltage divider formed by theresistors 92 and 94. PMOS transistors 116 and 120 are connected as acurrent mirror and operate as an active load to the bandgap referencedcomparator 82. The sources of the PMOS transistors 116 and 120 areconnected to the positive terminal 70. The gate and drain ofdiode-connected PMOS transistor 116 are connected to the gate of PMOStransistor 120 and to the collector of BJT 112. The drain of the PMOStransistor 120 is connected to the collector BJT 114 and forms theoutput 90 of the bandgap referenced comparator 82. The BJTS 112 and 114are sized differently with the BJT 112 having a larger junction areathan the BJT 114, and they are operated with the same current level dueto the current mirror formed by the PMOS transistors 116 and 120. TheBJTS 112 and 114 therefore have different current densities so thattemperature dependencies cancel out, ensuring a temperature independentbandgap voltage. In other embodiments, the bandgap referenced comparator82 may be configured differently or with different components to performsubstantially the same function.

The cascoded current source 84 pulls up the current-based statusindicator signal 50 to the positive terminal 70 when the output 90 ofthe bandgap referenced comparator 82 is on, that is, when the input 86of the bandgap referenced comparator 82 reaches the bandgap voltage. Thecascoded current source 84 includes a PMOS transistor 122 with thesource connected to the positive terminal 70 and gate connected to theoutput 90 of the bandgap referenced comparator 82. The cascoded currentsource 84 also includes a cascoded PMOS transistor 124 with the sourceconnected to the drain of the PMOS transistor 122, the gate connected tothe gates of the BJTS 112 and 114, and with the drain forming thecurrent-based status indicator signal 50. In one embodiment, the PMOStransistor 122 in the cascoded current source 84 and the PMOStransistors 116 and 120 in the bandgap referenced comparator 82 arematching transistors of the same species so that the PMOS transistor 122turns on at the proper voltage level and is not in a sub-thresholdconduction state. The cascoded PMOS transistor 124 is a high voltagetransistor, acting as a level shifter so that the current-based statusindicator signal 50 are ground referenced for the comparator 56.

The comparator 56 (FIG. 2) may include any suitable device fordetermining whether current is on or off in the current-based statusindicator signals 50, 52 and 54. In one embodiment, the comparator 56includes a current comparator for each of the current-based statusindicator signals 50, 52 and 54, with each current comparator comparingthe current level in the associated current-based status indicatorsignals 50, 52 and 54 against a reference current mirrored from anadjustable bandgap-based current source set at a very low current. Ifthe current through the current-based status indicator signals 50, 52and 54 is above the reference current, the comparator 56 indicates thatthe current is on.

During operation, the cell connection status detector 40 may be adaptedto operate in two modes, one for detecting overvoltage conditions andthe other for detecting undervoltage conditions, by varying the ratio ofthe input voltage applied to the input 86 of the bandgap referencedcomparator 82 and reversing the logic of the comparator 56 interpretingthe current-based status indicator signal 50. In a first mode, the cellconnection status detector 40 detects overvoltage conditions across thecell 14. For example, an overvoltage condition may be defined as 2Vacross the cell 14 between the positive and negative terminals 70 and66. If the base to emitter voltage VBE of the BJT 112 is 0.8V, and thevoltage drop across the resistors 92 and 94 below the BJT 112 is 0.4V,the bandgap referenced comparator 82 will turn on when the input 86reaches 1.2V. This may be achieved, for example, by selecting a topresistor 100 having a 4 MΩ resistance and a middle and bottom resistor102 and 104 having a combined 6 MΩ resistance. This applies a fractionof six tenths of the voltage at the positive terminal 70 to the input86, so that the input 86 is at 1.2V and will turn on the bandgapreferenced comparator 82 when the positive terminal 70 is at 2V. Thebandgap referenced comparator 82 and cascoded current source 84 will becutoff when the positive terminal 70 is below 2V in this case, and theonly current flowing will be through the voltage divider 80. The cellconnection status detector 40 may therefore be left in the overvoltagedetection mode during normal operation without drawing excessivecurrent. An overvoltage condition may occur for a variety of reasons,such as the use of the wrong type of battery charger that applies toohigh a voltage to the battery 12, or when two adjacent cells areinternally shorted in the battery 12, dumping all the voltage of the twocells into one input.

In a second mode, the cell connection status detector 40 detectsundervoltage conditions across the cell 14. For example, an undervoltagecondition may be defined as having less than 0.8V across the cell 14between the positive and negative terminals 70 and 66. Thus, thefraction of the voltage at the positive terminal 70 applied to the input86 is changed by closing the switch 110, and the logic of the comparator56 interpreting the current-based status indicator signal 50 is invertedso that the common reference status indicator signal 60 is turned onwhen the voltage at across the cell 14 is less than the undervoltagetrip point and the current in the current-based status indicator signal50 is off. The values of the resistors 100, 102, 104, 106, 92 and 94 inthe cell connection status detector 40 are selected so that the bandgapreferenced comparator 82 is turned on at the desired overvoltage trippoint with the switch 110 open in the overvoltage mode and turned on atthe desired undervoltage trip point with the switch 110 closed in theundervoltage mode. In the undervoltage mode, the bandgap referencedcomparator 82 is normally on and current is flowing through thecurrent-based status indicator signal 50, until the voltage across thecell 14 falls below the undervoltage threshold. Therefore, although thecurrent in the bandgap referenced comparator 82 and cascoded currentsource 84 is very small, in one embodiment the switch 110 is closed onlybriefly to detect undervoltage conditions and to latch the result fromthe comparator 56. The undervoltage detection may be performed just atstartup, or periodically, or continuously as desired. An undervoltagecondition may occur because of an open condition or bad connection to acell 14, or because of a discharged battery 12.

Turning now to FIGS. 4 and 5, another embodiment of the multi-cell powersystem controller 10 automatically detects the number of cells in abattery 12, allowing the same integrated circuit to be used with variousmulti-cell batteries without the need to configure the controller 10with configuration pins or pre-programmed non-volatile memory. Athree-cell controller 10 in a three-cell configuration is illustrated inFIG. 4, and a three-cell controller 10 in a two-cell configuration isillustrated in FIG. 5. Unused cell inputs 70 are shorted to V_(CC) 126at the top potential of the battery 12 as illustrated in FIG. 5, and thecontroller 10 automatically detects which cell inputs 70 are unused bythe battery 12. The controller 10 also provides configurable opencircuit detection at the cell inputs (e.g., 70).

As with previous embodiments, because the cell connection statusdetectors 40, 42 and 44 may be identical, the detailed description ofthis embodiment will be focused on one of the cell connection statusdetectors 40 illustrated in FIG. 6. Again, the input 70 from the topcell 14 acts as a positive terminal to the cell connection statusdetector 40 and the input 66 acts as a negative terminal. The cellconnection status detector 40 has a current mirror formed by a firstdiode connected PMOS transistor 130 and a second PMOS transistor 132connected between the positive terminal 70 and the negative terminal 66and acting as a voltage to current converter. The current through thecurrent-based status indicator signal 50 is proportional to the voltageacross the cell 14.

An enable switch 134 may be included in the cell connection statusdetector 40 in series with the diode connected PMOS transistor 130 toturn off the cell connection status detector 40 and conserve power whennot actively configuring the controller 10 or checking for open circuitsat the cell inputs (e.g., 70). Resistors 136, 140 and 142 may also beincluded in series and in parallel with the current mirror in the cellconnection status detector 40 to bias the PMOS transistor 130 as desiredand to limit current through the cell connection status detector 40.

The current level through the current-based status indicator signal 50is evaluated by the comparator 56 by comparing it against an adjustablereference current. The reference current may be generated in anysuitable manner, such as using a bandgap-based current source (e.g.,144). As with previous embodiments, a single bandgap-based currentsource 144 may be used in the comparator 56 for all current-based statusindicator signals 50, 52 and 54 by mirroring the reference current tocomparator devices for each indicator signal. The reference current maybe adjusted dynamically by an on-chip digital core in the controller 10or by an external host to automatically configure the controller 10 andto detect open circuits. The resulting common reference status indicatorsignals 60 may be latched for use by other devices.

The output current through the current-based status indicator signal 50may be derived by the expression in equation 1:

$\begin{matrix}{R_{i\; n} = {\frac{V_{1} - V_{GSPMOS}}{I_{1}M} + R}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where I₁ is the current through the current-based status indicatorsignal 50, M is the mirror ratio, V_(GSPMOS) is the gate to sourcevoltage of the diode connected PMOS transistor 130, R_(in) is theresistance of the input resistor 22 and R is the total resistance inseries with the diode connected PMOS transistor 130 across the cell 14.Corresponding equations can be developed for the currents through theother current-based status indicator signals 52 and 54. Generally thecell connection status detectors 40, 42 and 44 are configured with equaloutput current levels, although they may also be configured to generatevarious output current levels if desired.

A cell 14 is detected as present during automatic configuration of thecontroller 10 when the current I₁ from a cell connection status detector40 is greater than a reference current I_(REF1) to which the current I₁is compared in the comparator 56. The reference current I_(REF1) in thecomparator 56 is set at a level corresponding to a non-zero inputresistance, for example about 1 kΩ. This indicates that a voltage ispresent across the cell 14. If the positive terminal 70 is shorted toV_(CC) 126 as illustrated in FIG. 5, there will be no voltage across thecell between the positive terminal 70 and the negative terminal 66.

The cell connection status detector 40 may also detect an opencondition, where the input resistance (e.g., 22) is greater thanexpected. An open condition is defined herein as any resistance greaterthan a threshold resistance set to distinguish defective connectionsfrom normal operating resistances. This threshold resistance may be setat any level desired. For example, if the input resistors 22, 24 and 26are intended to be about 1 kΩ, the threshold resistance may be set atabout 250 kΩ. If the current I₁ through the current-based statusindicator signal 50 is less than the reference current Iref1 thatcorresponds with an input resistance of 250 kΩ, the comparator 56indicates that an open condition exists at the cell input 70. It maythus be seen that the cell detection and the open detection areimpedance level detections with different thresholds. The impedancelevel detection can be translated to a current level detection. Thedifferent thresholds can be adjusted by varying the reference current inthe comparator 56. Thus, the same cell connection status detector 40 canbe used both for determining the number of cells in the battery 12 andfor open circuit detection.

The operation of the controller 10 during automatic configuration isillustrated in FIG. 7 for a battery 12 having two 4V cells 16 and 20(see FIG. 5). When the controller 10 is first powered up or after apower on reset event 146, the enable switches (e.g., 134) are turned onbriefly in the cell connection status detectors 40, 42 and 44. Duringthis period, the current mirrors in the cell connection status detectors40, 42 and 44 are connected across the cells 14, 16 and 20 to sample thevoltage at the cell inputs 70, 66 and 64. Given the battery 12 of FIG. 5with two cells 16 and 20 connected and with the top cell input 70shorted, the inputs 64 and 66 will each have a voltage 150 and 152across them and the top input 70 will indicate that there is no voltage154 between the third cell input 70 and the second cell input 66. Thevoltages 150, 152 and 154 are converted to currents in the current-basedstatus indicator signals 50, 52 and 54, which are then interpreted inthe comparator 56 with the results stored in a latch as described above.The output of the comparator 56 during the automatic configurationprocess is illustrated in FIG. 8. When the controller 10 is firstpowered up or after a power on reset event 146, an automaticconfiguration signal 160 turns on the enable switches (e.g., 134) in thecell connection status detectors 40, 42 and 44. The automaticconfiguration signal 160 may be level shifted as appropriate to activatethe switches (e.g., 134). After the cell connection status detectors 40,42 and 44 have generated the currents in the current-based statusindicator signals 50, 52 and 54, the comparator 56 generatesground-referenced status indicator signals 60, one 162, 164 and 166 foreach of the current-based status indicator signals 50, 52 and 54. In theconfiguration of FIG. 5, the ground-referenced status indicator signals164 and 166 are asserted and the ground-referenced status indicatorsignal 162 is not.

After the number of cells is determined during the automaticconfiguration, the reference current 144 is adjusted by an internalmicroprocessor in the controller 10 or by an external host (not shown)to prepare the controller 10 for open circuit detection. During opendetection, the same procedure described above is repeated to detect afault condition and appropriate logic circuitry is used.

The controller 10 does not consume power once the initial determinationof the number of connected cells is made unless open detection isperiodically performed. Using the information latched from thecomparator 56, various parts of the controller 10 performing otherfunctions such as battery charging can be shut down, leading todecreased power consumption in the overall integrated circuit. Theaverage current consumed for open detection is extremely low as it is asampled data system. By using current mode signal processing, the highvoltages applied to the controller 10 by the battery 12 can be processedby low voltage digital circuits. Furthermore, the current in the diodeconnected PMOS transistors (e.g., 130) flow through the next cell in thestack which reduces overall current consumption.

A method for detecting cell connection status of a multi-cell battery issummarized in the flow chart of FIG. 9. The method includes converting avoltage across each cell of the multi-cell battery to a current togenerate a current-based connection status indicating signal for each ofthe cells (block 200), evaluating a current level of each of thecurrent-based connection status indicating signals (block 202), andgenerating a common reference status indicating signal for each of thecurrent-based connection status indicating signals based on the currentlevels (block 204). As described above, the common reference statusindicating signals are referenced to the same voltage level while eachcell of the multi-cell battery is referenced to a different voltagelevel. The method summarized in the flow chart of FIG. 9 is not limitedto any particular order of operation, and various portions of the methodmay be performed in any suitable order, whether in series, in parallel,or in some combination of the two. Furthermore, the flow chart of FIG. 9does not imply causal relationships or dependencies between the variousportions of the method, but is merely an example and summary of theoperation of one embodiment.

While illustrative embodiments have been described in detail herein, itis to be understood that the concepts disclosed herein may be otherwisevariously embodied and employed.

1. An apparatus for detecting cell connection status of a multi-cellbattery, the apparatus comprising: a plurality of battery cell inputs; aplurality of cell connection status detectors, each connected to abattery cell input, each of the plurality of cell connection statusdetectors having a current-based status indicator output; and at leastone comparator connected to the plurality of current-based statusindicator outputs, wherein each of the plurality of cell connectionstatus detectors floats in a different supply voltage range, and whereinthe at least one comparator is referenced to a lower voltage potentialthan at least one of the plurality of cell connection status detectors.2. The apparatus of claim 1, wherein each of the plurality of cellconnection status detectors comprises a bandgap referenced voltage tocurrent converter connected between a positive terminal and a negativeterminal, wherein the positive terminal and negative terminal compriseadjacent ones of the plurality of battery cell inputs, and wherein theat least one comparator is adapted to determine whether a current is onor off in the plurality of current-based status indicator outputs. 3.The apparatus of claim 2, wherein each of the plurality of bandgapreferenced voltage to current converters comprises: a voltage dividerthat is connected between the positive terminal and the negativeterminal; a bandgap referenced comparator that is connected to thevoltage divider, wherein the bandgap referenced comparator is adapted toturn on an output of the bandgap referenced comparator when the voltagedivider reaches a bandgap voltage; and a cascoded current sourceconnected to an output of the bandgap referenced comparator.
 4. Theapparatus of claim 3, wherein each of the bandgap referenced comparatorscomprise: a first diode-connected P-channel transistor connected to thepositive terminal; a first NPN BJT transistor connected to the diodeconnected P-channel transistor, the first NPN BJT transistor having abase connected to the voltage divider; a second P-channel transistorconnected to the positive terminal, wherein a gate of the secondP-channel transistor is connected to a gate of the first diode-connectedP-channel transistor; and a second NPN BJT transistor connected to thesecond P-channel transistor, wherein a base of the second NPN BJTtransistor is connected to the base of the first NPN BJT transistor, andwherein an emitter of the second NPN BJT transistor is connected to anemitter of the first NPN BJT transistor through a second voltagedivider.
 5. The apparatus of claim 4, wherein each of the cascodedcurrent sources comprise: a third P-channel transistor having a sourceconnected to the positive terminal and a gate connected to the output ofthe bandgap referenced comparator; and a fourth P-channel transistorhaving a source connected to a drain of the third P-channel transistorand a gate connected to the base of the first NPN BJT transistor,wherein the fourth P-channel transistor has a higher voltage rating thanthe third P-channel transistor.
 6. The apparatus of claim 3, whereineach of the voltage dividers comprise a first resistor, a secondresistor and a third resistor connected in series between the positiveterminal and the negative terminal, wherein the first resistor isconnected to the positive terminal and the bandgap referenced comparatoris connected to a node between the first resistor and the secondresistor, wherein each of the plurality of cell connection statusdetectors further comprises a fourth resistor switchably connectedbetween the positive terminal and a node between the second resistor andthe third resistor.
 7. The apparatus of claim 6, wherein each of theplurality of cell connection status detectors is adapted to detect anovervoltage condition at the associated one of the battery cell inputswhen the fourth resistors are disconnected from the voltage dividers andto detect an undervoltage condition when the fourth resistors areconnected to the voltage dividers.
 8. The apparatus of claim 1, whereineach of the plurality of cell connection status detectors comprises avoltage to current converter connected between a positive terminal and anegative terminal, wherein the positive terminal and negative terminalcomprise adjacent ones of the plurality of battery cell inputs, andwherein the at least one comparator is adapted to determine a currentmagnitude in each of the plurality of current-based status indicatoroutputs.
 9. The apparatus of claim 8, wherein the apparatus is adaptedto detect that a battery cell is connected to each of the plurality ofbattery cell inputs when an associated one of the current-based statusindicator outputs is greater than a first current level in a first modeof operation and to detect an open condition in each of the plurality ofbattery cell inputs when an associated one of the current-based statusindicator outputs is less than a second current level in a second modeof operation.
 10. The apparatus of claim 9, wherein the at least onecomparator comprises an adjustable reference current source, and whereinthe apparatus is adapted to generate the first current level in theadjustable reference current source in the first mode of operation andto generate the second current level in the adjustable reference currentsource in the second mode of operation.
 11. The apparatus of claim 10,wherein the at least one comparator comprises a single bandgap deviceand a plurality of current mirrors, one current mirror for each of theplurality of current-based status indicator outputs.
 12. The apparatusof claim 8, wherein each of the voltage to current converters furthercomprises: a first diode connected P-channel transistor connectedbetween the positive terminal and the negative terminal; and a secondP-channel transistor connected between the positive terminal and thecurrent-based status indicator output, wherein a gate of the first diodeconnected P-channel transistor is connected to a gate of the secondP-channel transistor.
 13. The apparatus of claim 12, wherein each of thevoltage to current converters further comprises a switch connected inseries with the first diode connected P-channel transistor between thepositive terminal and the negative terminal.
 14. The apparatus of claim13, wherein each of the voltage to current converters further comprisesa resistor connected in series with the switch and the first diodeconnected P-channel transistor between the positive terminal and thenegative terminal.
 15. A method for detecting cell connection status ofa multi-cell battery, the method comprising: converting a voltage acrosseach cell of the multi-cell battery to a current to generate acurrent-based connection status indicating signal for each of the cells;evaluating a current level of each of the current-based connectionstatus indicating signals; and generating a common reference statusindicating signal for each of the current-based connection statusindicating signals based on the current levels, wherein the commonreference status indicating signals are referenced to a same voltagelevel and wherein each cell of the multi-cell battery is referenced to adifferent voltage level.
 16. The method of claim 15, wherein convertingthe voltage across each cell to a current comprises applying a fractionof the voltage across each cell to a bandgap device, and whereingenerating the common reference status indicating signal comprisesasserting overvoltage indicator signals when current is on in thecurrent-based connection status indicating signals.
 17. The method ofclaim 16, wherein converting the voltage across each cell to a currentfurther comprises applying a different fraction of the voltage acrosseach cell to the bandgap device, and wherein generating the commonreference status indicating signal further comprises assertingundervoltage indicator signals when current is off in the current-basedconnection status indicating signals.
 18. The method of claim 15,wherein generating the common reference status indicating signalcomprises asserting a cell connected indicator signal when a currentlevel each of the current-based connection status indicating signals isgreater than a reference current level.
 19. The method of claim 18,wherein generating the common reference status indicating signal furthercomprises asserting an open circuit indicator signal when a currentlevel each of the current-based connection status indicating signals isless than a second reference current level.
 20. An apparatus fordetecting cell connection status of a multi-cell battery, the apparatuscomprising: a plurality of battery cell inputs; a plurality of cellconnection status detectors, each connected to a battery cell input,each of the plurality of cell connection status detectors having acurrent-based status indicator output; and at least one comparatorconnected to the plurality of current-based status indicator outputs,wherein each of the plurality of cell connection status detectors floatsin a different supply voltage range, and wherein the at least onecomparator is referenced to a lower voltage potential than at least oneof the plurality of cell connection status detectors, wherein each ofthe plurality of cell connection status detectors comprises: aswitchable voltage divider connected across the cell, the switchablevoltage divider being adapted to switch between a first resistance and asecond resistance at an output of the switchable voltage divider,wherein each of the plurality of cell connection status detectors isadapted to detect an overvoltage condition at the associated one of thebattery cell inputs when the switchable voltage divider is switched tothe first resistance and to detect an undervoltage condition when theswitchable voltage divider is switched to the second resistance; a firstdiode-connected P-channel transistor connected to the positive terminal;a first NPN BJT transistor connected to the diode connected P-channeltransistor, the first NPN BJT transistor having a base connected to thevoltage divider; a second P-channel transistor connected to the positiveterminal, wherein a gate of the second P-channel transistor is connectedto a gate of the first diode-connected P-channel transistor; and asecond NPN BJT transistor connected to the second P-channel transistor,wherein a base of the second NPN BJT transistor is connected to the baseof the first NPN BJT transistor, and wherein an emitter of the secondNPN BJT transistor is connected to an emitter of the first NPN BJTtransistor through a second voltage divider.